1. Field of the Invention
This invention relates to a radiation image signal acquiring method. This invention particularly relates to a radiation image signal acquiring method, wherein light emitted by a stimulable phosphor sheet, on which a radiation image of an object has been stored, during scanning with stimulating rays is detected, and wherein an image signal representing the radiation image of the object is thereby acquired.
2. Description of the Related Art
Radiation image recording and read-out systems utilizing stimulable phosphors have heretofore been known. With the radiation image recording and read-out systems utilizing the stimulable phosphors, radiation carrying image information of an object is irradiated to a sheet containing a stimulable phosphor (hereinbelow referred to as the stimulable phosphor sheet), and a radiation image of the object is thus stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored on the stimulable phosphor sheet during the exposure of the stimulable phosphor sheet to the radiation. The light emitted by the stimulable phosphor sheet is photoelectrically detected, and an image signal representing the radiation image of the object is thereby acquired.
For example, in the cases of mammography, in which a radiation image of a breast is recorded and read out, the scanning of a stimulable phosphor sheet, on which the radiation image of the breast has been stored, with the stimulating rays has heretofore been performed in a direction, along which an object image-carrying edge area carrying a radiation image pattern representing a chest wall side of the breast extends, the object image-carrying edge area being one of edge areas of the stimulable phosphor sheet. The scanning of the stimulable phosphor sheet with the stimulating rays has thus been performed heretofore in the aforesaid direction since a relationship between the position of a cassette, in which the stimulable phosphor sheet has been accommodated, and the position of the breast at the time of the recording of the mammogram and a relationship between the position of the cassette and the position of a radiation image read-out apparatus at the time of the loading of the cassette into the radiation image read-out apparatus for the readout of the radiation image from the stimulable phosphor sheet have been set, such that the aforesaid relationships may always coincide with predetermined positional relationships.
As described above, when the stimulable phosphor, which is contained in the stimulable phosphor sheet carrying the radiation image stored thereon, is exposed to the stimulating rays, the stimulable phosphor releases radiation energy, which has been stored in the stimulable phosphor, as the emitted light. The intensity of the light emitted by the stimulable phosphor reaches an approximately maximum light emission intensity quickly (e.g., within several nanoseconds) after the stimulating rays have been irradiated to the stimulable phosphor. However, after the irradiation of the stimulating rays to the stimulable phosphor has been finished, the light emission by the stimulable phosphor continues as after-glow. The light emission intensity of the after-glow becomes low with the passage of time.
Therefore, in cases where the light, which is emitted by the stimulable phosphor sheet during the scanning of the stimulable phosphor sheet with the stimulating rays, is detected, the light containing both the light emitted from an area of the stimulable phosphor sheet, which area is being exposed to the stimulating rays, and the after-glow emitted from an area of the stimulable phosphor sheet, at which area the irradiation of the stimulating rays has already been finished, is detected. In cases where an image signal is acquired with the detection of the emitted light containing the after-glow and utilized for reproduction of a visible image representing the radiation image stored on the stimulable phosphor sheet, sharpness of the reproduced visible image becomes low as the intensity of the after-glow mixed in the emitted light becomes high, i.e., as the after-glow components contained in the emitted light to be detected increase.
As a technique for suppressing the adverse effects of the after-glow described above, there has been known a technique, wherein a relationship between a peak optical intensity of the light, which is emitted by the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, and the light emission intensity of the following after-glow is approximately represented by a function, or information representing the relationship described above is stored previously as a data table, wherein the components representing the after-glow are removed from the image signal representing the optical intensity of the emitted light having been detected, and wherein an image signal representing the radiation image is thereby acquired. (The afore said technique for suppressing the adverse effects of the after-glow, wherein the components representing the after-glow are removed from the image signal representing the optical intensity of the emitted light having been detected, is described in, for example, Japanese Unexamined Patent Publication No. 10(1998)-232452.)
Also, in cases where a stimulable phosphor sheet has been subjected to a radiation image recording operation, and a radiation image of an object has been recorded on the stimulable phosphor sheet, an indirect radiation-exposed region and a direct radiation-exposed region are formed on the stimulable phosphor sheet. The indirect radiation-exposed region is the region having been exposed to radiation having been produced by a radiation source, which radiation carries image information of the object, such as a human body, (e.g., by passing through the object) and has attenuated energy. The direct radiation-exposed region is the region having been exposed to radiation having been produced by the radiation source, which radiation does not carry image information of the object (e.g., which has passed through areas other than the object) and has energy having not been attenuated. A different technique for suppressing the adverse effects of the after-glow described above has been proposed, wherein the indirect radiation-exposed region on the stimulable phosphor sheet is recognized, and wherein image readout is performed on only the indirect radiation-exposed region having been recognized. (The proposed different technique, wherein the image readout is performed on only the indirect radiation-exposed region, is described in, for example, Japanese Unexamined Patent Publication No. 62(1987)-018536.) Specifically, the stimulable phosphor located within the direct radiation-exposed region stores a high level of radiation energy, and the stimulable phosphor located within the indirect radiation-exposed region stores a level of radiation energy lower than the level of radiation energy stored in the direct radiation-exposed region. The proposed different technique described above, wherein the image readout is performed on only the indirect radiation-exposed region, aims at preventing the problems from occurring in that the after-glow having a high light emission intensity, which after-glow emanates from the direct radiation-exposed region after the direct radiation-exposed region has been exposed to the stimulating rays for the image readout, mixes into the light, which is emitted from the indirect radiation-exposed region and carries the image information of the object. More specifically, with the proposed different technique described above, wherein the image readout is performed on only the indirect radiation-exposed region, the stimulating rays having a low intensity is previously irradiated to the stimulable phosphor sheet, on which the radiation image of the object has been stored, and the light emitted by the stimulable phosphor sheet is detected. In this manner, a difference between the direct radiation-exposed region, which emits the light having a high light emission intensity, and the indirect radiation-exposed region, which emits the light having a light emission intensity lower than the light emission intensity of the light emitted from the direct radiation-exposed region, is recognized. Thereafter, the image readout is performed on only the indirect radiation-exposed region.
However, with the aforesaid technique for suppressing the adverse effects of the after-glow, wherein the components representing the after-glow are removed from the image signal representing the optical intensity of the emitted light having been detected, the problems occur in that complicated operation processing is required for the removal of the after-glow components from the image signal, and therefore the efficiency with which the image signal representing the radiation image is acquired is not capable of being kept high.
Also, with the aforesaid technique for suppressing the adverse effects of the after-glow, wherein the image readout is performed on only the indirect radiation-exposed region, the problems occur in that complicated processing is required for the recognition of the indirect radiation-exposed region, in that complicated control is required for the irradiation of the stimulating rays only to the indirect radiation-exposed region, and in that the apparatus cost is not capable of being kept low.
Therefore, there is a strong demand for a technique capable of easily suppressing the adverse effects of the after-glow.